Convection oven i.e. ovens in which gases are continuously circulated around to provide turbulent gas flow, are utilized for the curing of coatings on strip material such as strip sheet metal.
The circulating gases are heated so as to heat up the strip and the coating and produce a rapid baking or cure of the coating.
The solvents are vaporized in the process and are entrained in the gases. By suitable means of ventilation such solvent vapours must be kept below a certain level of concentration in the oven, in order to avoid the danger of explosion. In addition, to vent such solvent vapours untreated directly to atmosphere would cause environmental damage.
Accordingly it is the practice to provide continuous incineration of oven gases to oxidize the solvent vapours and render them harmless. In most cases such incineration is carried out in the exhaust stack from the oven, with the incinerated gases being simply exhausted directly to atmosphere. In some cases heat exchangers are employed in the stack to recover some of the heat values which would otherwise be lost up the stack.
In accordance with more modern practice however it has been found advantageous to carry out some of such incineration within the oven itself. In this way, oven atmosphere rich in solvent vapour can be transferred from one point in the oven, and then can simply be reintroduced into the oven through a zone inincinerator as solvent-free atmosphere, so providing part of the required ventilation to limit concentration of solvent vapours in the zone and providing the heat demands of the zone. One advantage is that the oven atmosphere is retained within the oven and does not have to be exhausted to atmosphere. This in turn means that less fresh air has to be introduced into the oven for the purpose of ventilation, and thereby reducing the fuel required for heating the incoming fresh air.
It has been known for some time that solvent vapours are evaporated largely in the first quarter to one third of an oven. During the remaining two thirds to three quarters of the oven, the amount of solvent vapours developed diminishes to zero so that as the strip passes through this part of the oven, the coating is simply being cured, with virtually no solvent vapours being evaporated at all.
Accordingly, the best engineering practice provides the greatest amount of "ventilation" ie. solvent free gas flow within the first quarter to one third of the oven. In this way, the solvent vapours being developed are rapidly entrained and carried away and mixed with oven gases. In this way, the concentration of such solvent vapours in the oven atmosphere is kept to within acceptable limits even in these zones of high solvent release.
Some of these solvent rich gases are transferred downstream to zones of lower or no solvent release, and in some cases, to balance this downstream gas flow, low solvent vapour gases were transferred upstream so as to maintain the ventilation within the upstream zones of high solvent release.
Zone incinerators are provided in the various zones for incinerating zone gases in each zone, and also for receiving gases transferred from other zones and incinerating them to provide solvent free high temperature gases in each zone.
In all of these systems however it has always been the practice that the gases being exhausted to atmosphere were exhausted from the zones of highest solvent release. This zone would usually be in the first one quarter to one third of the oven. In this way it was assumed that the high volume of solvent vapour in the zone of high solvent release was split between the various zone incinerators throughout the oven and the afterburner in the exhaust stack, and the latter portion, after passing through the stack afterburner could be discharged to atmosphere, substantially free of pollutants.
The extraction of solvent rich gases for incineration at the stack afterburner created serious problems. Environmental considerations require strict limits on the percentage of untreated solvent vapours in the exhausted gases being released to atmosphere. In order to maintain these limits, when supplied with relatively solvent rich oven gases, it was necessary to operate the stack afterburner at very high temperatures. Typically these temperatures would be in the range of thirteen hundred to fifteen hundred degrees fahrenheit. This required the use of special high temperature alloys, and high efficiency fuels. In addition it was essential to ensure that there was adequate oxygen in the gases being exhausted to maintain efficient levels of combustion in the afterburner.
In some cases it was possible to recover part of the heat created by the stack afterburner, but often such heating values represented a total waste in the system.
In many cases however it is not necessary to provide this extreme degree of treatment of solvent vapours. This is particularly true in the case of retro fit installations where an old inefficient plant is simply being upgraded.
In addition the cost in terms of fuel consumption, required for this extreme treatment, may be greater than any savings, and in areas where fuel is scarce this factor may rule out the use of an exhaust stack afterburner altogether.
In accordance with the present invention, all solvent laden gases generated in the oven whether from the high solvent release zones or not, are passed through zone incinerators within the oven, with the zone incinerators discharging the incinerated gases directly into their respective zones. Separation between zones of low and high solvent release is maintained by inducing a slight "migration" of oven gases from the low solvent to the high solvent zones. Fresh air, sufficient to maintain combustion in the incinerators is inducted into the oven, by exhausting oven atmosphere from the last one quarter to one third of the oven, where only a negligible portion of the solvent is released. The exhaust gas is discharged through the stack substantially free of untreated solvents without further incineration. No stack afterburner is required. In this way the calorific value of the solvent is released within the oven.